John Calvin Commentary 1 Corinthians 6:8

"Nay, but ye yourselves do wrong, and defraud, and that [your] brethren." — 1 Corinthians 6:8 (ASV)

But ye do injury. For this reason, we see why he has denounced them so bitterly—because such a shameful desire for gain was prevalent among them that they did not even refrain from injuring one another. He stated earlier, to expose the seriousness of the evil, that those are not Christians who do not know how to endure injuries. This, then, adds emphasis, based on a comparison: for if it is wrong not to bear injuries patiently, how much worse is it to inflict them?

And that your brethren. This further highlights the evil; for if those who defraud strangers are doubly blameworthy, it is terrible for a brother to be cheated or robbed by a brother. Now all of us are brothers who call upon one Father in heaven (Matthew 23:9). At the same time, if anyone acts dishonestly towards strangers, Paul does not excuse the crime; but he teaches that the Corinthians were completely blinded in treating sacred brotherhood as unimportant.